This invention relates to an improved buried zener diode, in particular one providing minimal adverse consequences caused by series resistances.
Parasitic series resistances in the terminals have plagued buried zener diodes which limit the performance of the device. The fact that the zener diode is buried beneath the surface of the wafer increases these series resistances. Furthermore, these resistance values and temperature coefficients can vary from device to device according to process variations. These values and temperature coefficients are also affected by variations in power supply voltages, causing further unpredictable changes in the series resistances.
One prior art way to overcome the series resistance problem is by reducing the current through the diode. The less the current, the less the undesirable voltage changes which result from these series resistances. However, low current results in higher zener noise, an undesirable side effect.
A buried zener diode of the prior art is shown in FIGS. 1 and 2. The undesirable series resistances R.sub.1 and R.sub.2 lie between the N+ region 10 and the P+ region 11. When current is passed through the device, voltage drops through the series resistance and results in the voltage between the terminals 10 and 11 being different from the actual intrinsic breakdown voltage of the zener diode.
One way used in the prior art to reduce the effects of these voltage drops is called a "force-sense buried zener". This type of zener, shown in FIGS. 3 and 4, employs three terminals, an N+ terminal 13, a P+ sense terminal 14 and a force terminal 15. The primary current flow is between force terminal 15 and terminal 13, while the high resistance path flows to sense terminal 14 to minimize the adverse effects of the series resistance. However, even in this solution, there is still some resistive coupling between force terminal 15 and sense terminal 14, so the effects of the series resistances are not completely eliminated.
In applications where very low noise, low temperature drift and high power supply rejection is required, but where there are likely to be changes in the series resistances caused by process variations or temperature changes, these prior art solutions are insufficient. This invention provides a buried zener diode where the disadvantage created by these series resistances is almost completely eliminated.
Briefly, the improved zener diode of this invention comprises a semiconductor wafer having a primary surface and a zener diode having a PN junction buried beneath that surface. A pair of spaced P+/N+ contacts for the diode are located on the surface of the wafer, the first located on a first region of the surface and the second located on a second region of the surface laterally spaced from the first, the contacts establishing a current path through the diode across the PN junction.
The device of the invention also employs what is termed a "replica device", formed in the same semiconductor wafer, which generally replicates the physical dimensions of the zener diode. The replica device has a pair of spaced surface contacts with a replica current path between them having substantially the same on-resistance as the current path of the zener, the replica current path replicating the current path through the zener except that it has no PN junction in the current path.
Finally, the diode of the invention has a means coupling the replica device with the diode in a manner such that the voltage across the replica device is subtracted from the voltage across the diode to generate a difference voltage whereby, when current is simultaneously passed through the diode and the replica device, this difference voltage will be substantially independent of changes in the series resistance of the anode/cathode current path of the diode caused by variations in the wafer manufacturing process and by changes in the ambient temperature or applied voltage at which the diode is being used.